The invention is directed to a delay line for transit time amplifier tubes, which has a filter characteristic with negative dispersion in the fundamental wave of the longest wave pass band, i.e. the operating band, and is formed from a hollow conductor provided with successive transverse walls, each of which is provided with an electron beam opening and a coupling slot. An attenuating path which is provided with a lossy material and in the area of which at least one of the conductor dimensions which determines the two cutoff frequencies of the operating band, is so enlarged that, in the attenuation path, at least one of the two cutoff frequencies of the operating band lies at least at the same distance from the band center as in the remainder of the delay line. A line of this general type is known from German Pat. No. 2,400,331.
The patent referred to is concerned, in particular, with a type of line whose operating band is determined by the dimensions of the line cells and whose attenuation path is so designed that it fundametally affects only the magnetic field of the line wave (cavity line with current attenuation). In a line of this type, assuming equal dimensions along the entire length of the line, in the attenuating portion in particular, the lower cutoff frequency is displaced towards higher values so that undesired reflections readily occur during operation in the vicinity of the .pi.-point. The patent thus proposes that the cell diameters or the length of the coupling slots be increased.
If the attenuation path influences the magnetic field of the line wave to a lesser extent than the electric field (field attenuation), then the attenuating elements will influence the dispersion characteristic of the delay line in an entirely different manner. In this case the attenuation component is well matched at the .pi.-point, whereas disturbing oscillations are generated close to the 2 .pi.-point.